The invention relates generally to heat exchangers and more particularly to the pre-treatment of tubes for such heat exchangers.
The once-through steam generators or heat exchangers, associated with nuclear power stations and which transfer the reactor-produced heat from the primary coolant to the secondary coolant that drives the plant turbines may be as long as 75 feet and have an outside diameter of about 12 feet. Within one of these heat exchangers, tubes through which the primary coolant flows may be no more than ⅝ inch in outside diameter, but have an effective length of as long as 52 feet between the tube-end mountings and the imposing faces of the tubesheets. Typically, there may be a bundle of more than 15,000 tubes in one of these heat exchangers.
In the construction of a once-through steam generator, a plurality of these small diameter long length tubes are configured in a square array where they are welded at their top and bottom ends to a tubesheet to maintain this array in the once-through steam generator.
The original once-through steam generators were fabricated using a sequence where tubes, prior to welding to both tubesheets, were individually electrically heated such that cooling of the hot tubes after welding to the tubesheet resulted in tensile strains. This fabrication method is not recommended since in the thermal method of tube prestraining used on the original once-through steam generator the tubes were heated individually until the desired thermal strain was achieved and then seal-welded in place. Thus, for the first seal welded tubes, the desired prestrain was achieved exactly. As the procedure progressed, the previously welded tubes cooled and started to load the secondary shell and the tubesheets. In response, these components deflected in the direction of the load and effectively decreased the length of the subsequently welded tubes. This mechanism introduced an unwanted, uncontrolled and undefined tensile strain in these tubes. Excessive tensile stress was detrimental to the tube life. In addition, thermal prestraining is an expensive and time consuming process.
Since both the tubes and the shell of the once-through steam generator are restrained by the tubesheets at both ends, interaction stresses develop during operation due to the relative deformation of the steam generator shell and the tubes. These interaction stresses come from several sources. (1) Both the primary and secondary pressures elongate the secondary shell of the vessel between the two tubesheets; (2) the combined action of the primary and secondary pressures changes the tube radius which, in turn, causes a length change of the tube (xe2x80x9cPoisson effectxe2x80x9d), or a stress from resisting that change; (3) the tube temperature varies along its length and is different from the lengthwise temperature distribution of the secondary shell. This causes differential expansion of the two; (4) the tubes have a higher coefficient of thermal expansion than the secondary shell which causes a differential expansion; (5) tubesheet bowing, created by primary and secondary pressures combined with induced shell and head deflection loads; and (6) the tube preload introduced during manufacturing.
In the case of the once-through steam generator, tube buckling is caused by deformation controlled loads and thus is not a catastrophic primary stress failure mode. However, analysis of a tube shows that tube touching would occur very soon after the tube assumed a bowed shape. Therefore, the load which causes tubes to touch is considered as the limit load on the tube in compression.
A slight manufacturing tube prestrain of about xe2x85x9 inch over the length of the tube is considered beneficial to reduce compressive loads on the tubes under all operating conditions. This has the added benefit of preventing stress softening and the resultant reduction in tube natural frequency for flow induced vibration considerations.
In view of the foregoing it is seen that an improved method of prestraining the tubes of a once-through steam generator was needed which would not subject the tubes to interaction stresses during welding.
The present invention solves the prior art once-through steam generator or heat exchanger assembly problems and other problems by providing a method of prestressing the once-through steam generator tubes in which the tubes of the once-through steam generator are prestrained to the desired level using the hydraulic expansion of the tubes in the tubesheet. Prior to performing the tube joint hydraulic expansion, both ends of the tubes are welded to their respective tubesheet. The subsequent tube radial expansion within the hydraulic expansion zone creates the desired axial preload. It has been demonstrated both analytically and experimentally that tensile stresses are developed during the hydraulic expansion of tubes which are restrained at both ends. The obtained stresses are of the desired magnitude to increase margin to buckling and increase tube natural frequency to thus increase the margin to detrimental flow induced vibration.
Optimized selection of the final total tube stress is controlled by controlling the length of hydraulic expansion in the upper tubesheet. The main advantage is that the developed prestrain is independent of the tube load prior to the expansion. Therefore, the achieved pre-set of a given tube will be independent of the state of the other tubes resulting in the desired uniform foreshortening of each tube.
A tensile prestrain of xe2x85x9 inch over the tube length will assure that all tubing stress limits will be met and the tubes will be at a very low tensile stress of approximately 3 ksi during full power operation. This tensile prestrain is achieved by controlling the hydraulic expansion process.
In view of the foregoing it will be seen that one aspect of the present invention is to provide a prestraining method for once-through steam generator tube assemblies which will be constant for all the individual tubes.
Another aspect is to provide a prestraining method for once-through steam generator tubes once they are assembled into a once-through steam generator tube array.
These and other aspects of the present invention will be more fully understood from the following description of the invention when considered along with the accompanying drawings.